Typically, when a system is being clocked at a specific frequency, it is desirable that the frequency be very clean (e.g., a straight line in a spectrum). However, putting all the energy for the clock signal at one point in the spectrum introduces electromagnetic interference (EMI) concerns. For instance, it is easy to interfere with other systems or components that are sensitive to a particular frequency because the energy is concentrated in a narrow frequency band.
In order to control EMI between systems and/or components, the Federal Communications Commission (FCC) limits the energy a signal can emit at a point in the spectrum. The FCC measures EMI by using a spectrum analyzer to measure the peak energy at any one point or band in the spectrum. In a spread spectrum, a signal is swept back and forth across the spectrum, dwelling at a particular FCC band for a short period of time, reducing the power measured.
Typically, low frequency modulation is used for spreading the frequency and therefore the energy across several bands. Low frequency modulation is advantageous because it allows for any downstream phase locked loop (PLL) that is referencing the clock signal to track the clock signal where the frequency modulates slow enough to be followed. However, the downstream PLL will build up phase error in attempting to follow the low frequency. Phase error at the input indicates that the PLL must move faster to track the clock signal. When designing a system in which the downstream PLL will follow a spread spectrum clock signal, a delay is built into the system timing budget.
Synchronous systems with downstream PLLs having a bandwidth too low to track the spread spectrum signal will lose synchronization. Frequency modulated clock signals lead to downstream PLL tracking error which, when large enough, may cause system timing failures. Downstream tracking error occurs because the downstream PLL only changes its frequency when there is a phase error at its input. Therefore, to follow or track a signal that is changing frequency, a phase error occurs that is proportional to the frequency slew rate when modulation is within the downstream PLL bandwidth. However, it is ignored when the modulation is outside the PLL bandwidth.
It has been proposed to provide wider frequency modulation in order to spread the energy over more measured frequency bands using digital circuitry. However, digital circuitry for high frequency spread spectrum modulation presents a number of drawbacks. For one, the proposed digital circuitry requires a considerable amount of circuit area. As circuits continue to shrink in size, it is important that components require minimal circuit area. Furthermore, digital circuitry provides limited granularity or resolution, which limits the performance of current frequency spread spectrum modulation techniques.